Thin films of magnetic materials are important in mass data storage technology. Today's best “hard drives” are able to store approximately 100 Gbytes of information on a 3.5 inch (double-side) disk. Such a density corresponds to 4.2×1010 bits per in2 or a bit size of about 120 nm. Each bit is a magnetic domain, often a particle, capable of being magnetized in a certain direction for future readout. Magnetic disk technology (and other areas, including VLSI fabrication with ever decreasing line widths) faces some “physics-dictated” (rather than technique-dictated) limits. Specifically, when magnetic domains become too small, any alignment of spins by an external magnetic field is lost when the field is removed. At this superparamagnetic/ferromagnetic limit (which corresponds to a characteristic size of about 10 nm for Ni at room temperature) because the domains/particles are no longer able to sustain a permanent magnetic dipole (ferromagnetism), the material is not useful for magnetic storage.
One method to push the superparamagnetic limit beyond the 10×1010 bits per in2 barrier is to prepare “perpendicular” media, where anisotropic bits are magnetized perpendicular to the surface. Such highly anisotropic high aspect ratio particles exhibit perpendicular ferromagnetic behavior at diameters less than a spherical particle of the same diameter. This property permits a denser packing of ferromagnetic particles in the plane of the recording medium (i.e. a greater storage density). Other storage density advantages accrue from the fact that the particles will be also uniform in size and distribution. Uniformity in size means that no particle will be below the ferromagnetic limit.
Noteworthy advantages will be realized from the readout perspective. Small currents are induced in the readout head as it passes over magnetized bits. Relatively uniform spacing helps to ensure uniformity in readout current. The read head will encounter one identical particle, with predictable response, at a time, instead of various-size clumps of particles. A final significance of the disclosed magnetic material is also related to read-out: in order to be able to read individual bits, the read head must approach the disk surface as closely as possible, a requirement which is, itself, a challenge in lubrication (to prevent crashes). Anisotropy in the read head response and in the magnetic field of the particles are refinements which permit detection of closely-spaced bits. Rodlike particles, oriented and magnetized as described below, provide enhanced magnetization in the direction perpendicular to the disk surface, which facilitates readout. It is estimated that the advantages above, when taken in totality, will lead to an improvement in storage density of at least a factor of ten.
An array of uniform, high aspect-ratio rods has many potential applications other than magnetic storage. It is extremely difficult to produce uniform “columns” by photolithography. Tall features are subject to undercutting during the etching phase. While hairlike nanostructures, such as carbon nanotubes, may be grown from surfaces, they are not arranged in an array pattern. In addition, the nanorod arrays herein disclosed may be used as “masters” for preparing arrays of holes in softer materials, either by microcontact stamping or by molding (e.g. with rubbery polymers such as polydimethylsiloxane). Such applications illustrate further uses of the invention.